Alright, here's an idea for you along with a quick sketch. With this one, you'd cut a thin access line in the bottom of your frame, slide a plate with nuts welded into it inside the access line, and then weld the bottom plate to the bottom of your frame.
This will not weaken the frame, will allow you two solid mounts, and will keep you from dropping the tank.
The downside is that it might take just as long to fabricate the pieces as it would to drop the tank but if it's cold out, you'll be on your back for much less time.
For the front view of the drawing, that is in relation to you looking at the side of the truck. To make it easier, imagine you're holding the completed piece in your hand as you're facing the side of the truck. You would cut a slit in the frame, slide the piece up and into the slit, and then weld the bottom of the plate to the frame. Before welding though, I'd drill your holes through the frame and make sure they match up. If not, you can always elongate the holes and use a stout washer.
Does my explanation make any sense at all or have my descriptive skills failed me?